Normal alpha olefins (NAO) are produced commercially using ethylene and trialkyl aluminum compounds. The preferred trialkyl aluminum compound is triethyl aluminum (TEA). In this oligomerization or "growth" reaction, a hydrocarbon group, for example, an alkyl group on an aluminum atom containing n ethylene units, can add an ethylene molecule to become an alkyl group of n+1 ethylene units. Different alkyl groups on the aluminum atom can contain different numbers of ethylene units. In order to distinguish them from the original trialkyl aluminum catalysts, the products of this oligomerization reaction will be referred to herein as "tri(higher)alkyl aluminum compounds."
The tri(higher)alkyl aluminum compounds are subjected to a transalkylation (sometimes called a "displacement") reaction which occurs concurrently with the oligomerization reaction, thereby forming alpha olefins. This transalkylation consists of two steps. These are, first, thermal decomposition of the tri(higher)alkyl aluminum compounds into an aluminum hydride and alpha olefin(s), followed by rapid reaction of the hydride with the excess ethylene in the reaction mixture to regenerate an ethyl group on the aluminum which can start another oligomerization cycle.
The resulting mixture comprises alpha olefins, triethylaluminum, and some amount of residual tri(higher)alkyl aluminum compound(s) which have not undergone the transalkylation reaction. This mixture is subjected to caustic treatment (hydrolysis) to decompose the triethylaluminum compounds. Unfortunately, this hydrolysis also decomposes the residual tri(higher)alkyl aluminum compounds, which typically results in a product containing about 1 to 1.5 weight percent, based on the weight of the product, of undesired saturated alkanes ("saturates" or "paraffins") in the product, rather than a product which contains only the desired alpha olefins. Because these saturates have boiling points close to the desired alpha olefins, they are very difficult to remove from the product by, e.g., distillation.
The resulting mixture is separated into an aqueous and an organic phase, and, after settling, the organic phase is water-scrubbed (to remove residual caustic and aluminum compounds) and distilled to give the various even-numbered alpha olefin cuts from C.sub.4 up through C.sub.30 and higher. The normal alpha olefins produced, particularly the C.sub.12, C.sub.14, and C.sub.16 alpha olefins, have utility for the production of detergents. The C.sub.4, C.sub.6 and C.sub.8 olefins are useful for preparing linear low density polyethylene. The C.sub.10 olefin can be converted to oligomers useful as synthetic lubricants. As noted above, the olefin cuts will typically contain approximately about 1 to 1.5 weight percent paraffin, the paraffin being formed in the caustic hydrolysis step. In recent years, products containing lower paraffin contents have become available using other technologies. This has put pressure on the industry to upgrade its product.
Processes for making normal alpha olefins are disclosed in several U.S. patents. U.S. Pat. No. 2,781,410, issued Feb. 12, 1957 to Ziegler et at., discloses a process for the polymerization of ethylene to butene, hexene and higher olefins by contacting ethylene and a trialkyl aluminum compound. This causes the aforementioned "growth" reaction. It is disclosed that the range of temperature of "genuine catalysis" can be extended in the downward direction if "to the main catalysts aluminum trialkyl, nickel, cobalt or platinum is added as an activator."
U.S. Pat. No. 2,978,523, issued Apr. 4, 1961 to Coyne et at., discloses that prior processes for making higher olefins from lower olefins had involved the reaction of a trialkyl aluminum compound and ethylene to form the oligomerization product. After forming the product, it was heated in the presence of an additional quantity of ethylene and a finely divided metal catalyst, such as finely divided nickel. Coyne et at. disclose that these metal catalysts can include nickel, cobalt, palladium, and certain iron compounds. Their preferred catalyst is nickel or a nickel compound which will react with the trialkyl aluminum. Finely divided nickel metal, Raney nickel, nickel acetylacetonate, and nickel naphthanate are specifically disclosed. It is disclosed that the amount of catalyst used can vary greatly, and when the preferred catalyst is employed, the amount used may vary from about 0.001 to 0.1 percent based on the weight of the oligomerization product present.
U.S. Pat. No. 3,035,104, issued May 15, 1962 to Harvey et at., discloses that a trialkyl aluminum oligomerization product can be reacted with ethylene in the presence of about 0.01-1.0 tool percent of finely divided iron, preferably about 0.1 to 0.5 mol percent and about 0.001 to 0.01 mol percent, preferably about 0.001 mol percent of finely divided nickel for a time sufficient to displace the alkyl groups combined with the aluminum. The amounts of catalyst are based on the oligomerization product feed.
U.S. Pat. No. 3,277,203, issued Oct. 4, 1966 to Keehan et al., discloses the displacement of olefins from aluminum oligomerization product at elevated temperatures. In one displacement method, ethylene is reacted with aluminum alkyl oligomerization product in the presence of a finely divided metal catalyst, such as finely divided nickel. It is further disclosed that, while the catalyzed displacement reaction is very effective in providing the desired olefins, the use of a catalyst has disadvantages in that the catalyst is difficultly removable from the aluminum alkyl and may prevent the reuse of the latter material in the oligomerization process. Also, in the separation of olefins formed in the displacement reaction from the aluminum alkyl, for example, by distillation, the presence of the catalyst often produces undesirable reactions, such as reverse displacement and isomerization of olefins.
U.S. Pat. No. 3,363,021, issued Jan. 9, 1968 to Tucci, discloses a process for removing an aluminum trialkyl from a mixture which contains hydrocarbons and said aluminum trialkyls by subjecting the mixture to hydrolysis with water to convert the aluminum alkyls to the corresponding aluminum hydroxides, and the treated mixture can then be subjected to distillation to recover the hydrocarbons. As an alternative to this process, Tucci discloses a process which comprises treating the mixture with an ether, an amine or a sulfine to form a complex between the aluminum trialkyl and the ether, amine or sulfine, treating the complex with sodium, potassium, rubidium or cesium fluoride, cesium chloride or a complex of an aluminum trialkyl and one of such alkali metal halides, thereby freeing the ether, amine or sulfine from the first-named complex and forming a resulting complex between the first-named aluminum trialkyl and one of the defined alkali metal halides.
U.S. Pat. No. 3,482,000, issued Dec. 2, 1969 to Fernaid et al., discloses a process for the polymerization of ethylene to normal and branched alpha olefins in the presence of an organometallic catalyst, such as triethyl aluminum. Fernald et al. state that a high selectivity toward normal alpha olefins is achieved by performing the reaction in a tubular reaction zone wherein the amount of polymer increases throughout the length of the reactor tube. The reaction temperature is between about 180.degree. C. and 240.degree. C., there is from about 1.times.10.sup.-4 to about 1.times.10.sup.-2 mols of catalyst per mol of ethylene, and the polymerization proceeds until there is a conversion of at least about 30 percent of said ethylene to polymer product.
U.S. Pat. No. 3,510,539, issued May 5, 1970 to Fernald et at., discloses a process for the production of alpha olefins from ethylene in the presence of trialkyl aluminum catalyst and solvent in a tubular reactor immersed in a bath of heat exchange fluid wherein no catalyst and solvent are added to the ethylene until the ethylene is preheated to full reaction temperature in an upstream portion of the tubular reactor, and the reaction then occurs at a constant temperature in a downstream portion of the tubular reactor.
U.S. Pat. No. 3,702,345, issued Nov. 7, 1972 to Fernaid et al., discloses a process wherein ethylene is treated with an aluminum hydrocarbon to obtain a product predominating in normal alpha olefins. The process involves heating said product at an increased temperature level for a short period of time prior to recovery of the normal alpha olefins.
U.S. Pat. No. 4,918,254, issued Apr. 17, 1990 to Diefenbach et al., discloses a nickel-catalyzed displacement reaction in which the alkyl groups in trialkyl aluminum are displaced by alpha olefins in the presence of a nickel catalyst. The displacement is said to be fast and the catalyst is then poisoned with a catalyst poison such as lead to prevent undesired reactions such as isomerization of alpha olefins to internal olefins or dimerization to vinylidene olefins. The amount of nickel required to catalyze the reaction is said to be very low, on the order of parts per million (ppm). A useful range is said to be about 1-100 parts by weight nickel per million parts of reaction mixture. A preferred range is said to be 2-20 ppm, and a more preferred concentration is said to be 2-10 ppm.
A process has now been discovered which produces normal alpha olefins having lower paraffin content while at the same time resulting in minimal or substantially no isomerization of the alpha olefins.